Exceeding the WUI Code: Home Design in Wildfire-Prone Areas

Recent fire events in California have caused many homeowners to embark upon better understanding ways to defend their homes and property against the risks of wildfire. California’s building codes (CBC) currently prescribe certain requirements for homes built within the Wildland Urban Interface, or W.U.I. Yet in many instances homes, even new homes built to current CBC standards were destroyed, leading many constructing within the WUI to ask themselves if they should build in excess of current codes.

California’s WUI Code
To varying degrees, the heavy losses of lives and property resulting from recent fire events have all inevitably been associated with the wildland–urban interface (or WUI), the zone of transition between unoccupied land and human development. Chapter 7 of the California Building Code, also known as the wildland urban interface (WUI) Code, implemented in 2008 sets out requirements for building in areas deemed susceptible to wildfires. It codifies, which is to say mandates and proscribes, certain prescriptive methods for implementing enhanced fire-resistive construction techniques and technologies.

Yet anecdotally, in recent wildfires under certain circumstances even new homes built within the strict regulations of the WUI Code apparently succumbed and were destroyed. In fairness, this does not mean that the WUI Code failed these homes; apparently many of the buildings destroyed were lost to urban conflagrations, building-to-building firestorms not contemplated nor therefor within the purview of the WUI Code.

Even so, anyone building a new home, or especially rebuilding, within the WUI interface might ask themselves way in which the can further protect themselves, even in excess of the WUI Code.

Thinking Strategically
If substantively at risk to wildfire, part of the key to your home’s survival is to understand the way in which wildfire may attack the building. Wildfires are driven by strong winds they may reach peak temperatures in seconds, and then may pass your site and be gone within minutes, as soon as all the standing fuel is consumed. If designed to withstand this massive but brief onslaught, your home has a better chance of survival.

Site Layout
The first strategy is to get a many feet of distance and incombustible material between you and the wildlands using patios, driveways, or low-growing fire-retardant plants. The Fire Code sets this distance at 100 feet. That said, it has been noted anecdotally that some victims of recent fires, though willing, could not set up the prescribed site clearance because of overgrowths occurring on their neighbor’s land.

Wildfire can attack the house from any direction, but an up-slope running fire is by far the most dangerous and so this is the yard area most in need of fastidious site clearing. Further, if your resources to invest in systems to protect the building itself is limited, you might consider this to be the flank of the building at which to devote the most resources.

Ignition Resistant Roofs
The WUI Code admits of up to Class-B rated roofing materials. A more conservative approach might be to go with Class-A (non-combustible) roofing materials, which include metal roofing slate, concrete tile, and spanish tile roofing. The WUI code provides that any gaps or apertures within the roofing system “be firestopped with approved materials or have one layer of mineral-surfaced nonperforated cap sheet installed over combustible decking”.

In the special case of spanish tiles, at the roof edge the open ends present a target which sometimes is closed with a only a sheet metal closure. A safer bet is to specify endcap-closed tiles, and the safest approach is the traditional method of fully filling any voids with mortar. If the tiles are irregular, leaving any spaces between the tiles these gaps should be ‘fire-stopped’ (i.e. patched with mortar) to limit embers from getting under the roof covering.

Window Protection:
Windows are perhaps the weakest link in defending buildings against wildfires.

Radiant heat alone from severe fire can shatter glass or ignite combustibles within the building, even without the flames actually engaging it’s exterior. For this reason under WUI Code the minimum standard is double glazing with tempered glass on the exterior.

More fail-safe solutions include roll-down metal fire doors above the openings, released automatically by fusible links. They will protect windows and sliding glass doors even if they are left standing open. In the case of non-operable windows, in lieu of fire shutters, there are many kinds of wire glass or fire safety glass that hold together even if cracked by heat.

Doors:
Solid core wood doors are usually rated at 20 minutes of fire protection, but instead for extra safety consider a metal core door faced in any material you like, and for added safety use a metal jamb. Garage doors are especially susceptable, so consider a metal panel door with an automatic fusible link closure. Be sure it is especially tight fitting to prevent the wind from sliding a burning brand into any gaps, head, jamb, and sill.

Louvers and Vents:
Houses have lots of other holes in them that need to be protected.

In the case of attic ventilation at the eaves, the WUI code identifies that within the WUI interface conventional under-eave soffit vents need to include either 1/8” wire fabric mesh, specify to intumescent (fire-reactive) materials, or implement pop-up roof vents in their place. Of these, in the event of a firestorm passing quickly, intumescent undereave vents, specified properly, may offer the best approach.

In the case of attic ventilation at the ridge, WUI Code stipulates 1/8″-1/4″ wire mesh screening at the ridge vent. A more conservative approach might be to implement intumescent protection, although no such product appears to be available as of this writing.

WUI code does not speak to bathroom, dryer, and kitchen vents. A more conservative approach would be to specify bathroom, dryer, and kitchen vents having automatic back-draft dampers and fire-rated assemblies where they penetrate the exterior skin.

Plastic plumbing vents where they penetrate the roof should be avoided in favor of metallic vents. If plastic plumbing vents are unavoidable and in any event, metal sleeves and hoods should cover plastic plumbing vents where they penetrate the roof.

Infrastructure Systems:
Water: Unless you are on a city water main, anticipate that water supply to the property will be unreliable. Therefore consider to maintain and mark an emergency water supply such as a community water/hydrant system, a cooperative emergency storage tank with neighbors, or an on-property water storage tank of at least 2,500 gallons. Create easy firefighter access to your closest emergency water source.

Sprinklers: Consider implementing a wildfire sprinkler system. A mimimal system would protect the building proper i.e. roof, raised terraces, and decks. A more robust system will include this plus yard sprinklers for perimeter defense. Components of the system would include an on-site water tank, fire hydrant hook up, and an emergency generator plus pump to keep the system charged.

Electrical power: Assume that at the worst possible moment your house will probably loose both electric power and water pressure. Consider installing a generator that will automatically power both exterior lights plus any pumps feeding the sprinkler system.